4. Construct a Triangle Given Its Circumradius, Inradius and a Vertex Angle

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This Demonstration constructs a triangle given its circumradius , inradius and the angle at .



Draw a circle with center of radius and a chord of length . Let be the midpoint of and let be an endpoint of the diameter perpendicular to .

Step 1: Draw a line parallel to at distance from intersecting at .

Step 2: Draw a second circle with center through and . Let be one of the points of intersection of and .

Step 3: Let be the intersection of and .

Step 4: Draw the triangle and its incircle.


Theorem: Let . Then .

Proof: In the right-angled triangle , the hypotenuse length is and the leg , so ). Since the arc equals the arc , bisects . The distance of to is , so is the incenter of .


Contributed by: Izidor Hafner (August 2017)
Open content licensed under CC BY-NC-SA




[1] D. S. Modic, Triangles, Constructions, Algebraic Solutions (in Slovenian), Ljubljana: Math Publishers, 2009, p. 82.

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